The present invention determines, provides, and applies a precise GPS-standardized time base synchronization representation for the signals transmitted from the base transceiver stations (BTSs) of a cellular wireless communications system (WCS), for which the BTSs are not inherently and jointly synchronized to a common time base standard. In particular, the BTSs of currently primary interest are the cellular wireless transceiver stations serving routine or typical operations under an ETSI- or 3GPP-specified WCS, including those implemented for GSM and UMTS service.
As realized and noted in the background art for wireless communications systems, the timing of a 3GPP-specified BTS's transmitted signal is managed to provide transmitted signal frequencies that are within specified acceptance criteria of the nominal system-defined communications-channel frequencies. The time base tolerance for the WCS facilities is intended to permit acceptable communications performance involving any independently manufactured and distinctly compliant BTS and mobile station/user equipment (MS/UE). However, under the 3GPP specifications, neither the BTSs nor the MS/UEs must be jointly synchronized in common to any worldwide or area-wide time base standard. The compliant time base clock or oscillator for each BTS is specified to operate within the acceptable criteria for the nominal oscillator frequency, but otherwise operates independently from that for any other BTS. The compliant MS/UE time base clock or oscillator is also required to independently operate within specified criteria, to facilitate initial acquisition of the BTS downlink channel transmissions. Thereafter the MS/UE synchronizes itself to its reception of the serving BTS-transmitted frequency and communications frame synchronization time base, and then applies any received command for a timing-advance (TA) parameter relative to this detected time base. Thus inter-BTS co-synchronization is not required for the 3GPP wireless communications.
An objective of the present invention is to provide the technology for automated determination of the precise GPS-based time synchronization or “time tag/label” for specific instants, e.g., the digital frame boundaries, in the downlink signals transmitted by the wireless communications BTSs. In its enhanced performance contrasted with descriptions of the background art, the present invention achieves high accuracy in the GPS-based BTS-timing characterization through the evaluation and analysis of the downlink communications signals transmitted by the BTS. In accord with the applicable technical specifications for the “digital” WCS communications protocols, the digital data communicated between a BTS and the mobile units that it serves are organized into successive communications “frames,” with each frame comprising sets of “overhead” or “control” bits and the communicated “message” bits constituting the communications of interest to the served parties. These communications frames and their encapsulated data bits are transmitted at the specified data transmission rate and are modulated upon the specified transmission-frequency carrier in accord with the time base standard or oscillator that drives the electronics of the transmitting unit(s).
In a GSM or UMTS WCS, the time base for each individual BTS must meet a specification standard for tolerance around a nominal frequency, but otherwise the individual BTS timebases may and typically do operate independently, without inter-station synchronization. To enhance the services enabled through such synchronization, the technology of the present invention provides the precise determination of the temporal relationship between the BTS's self-generated time base/clock and the worldwide GPS time base, in a manner to precisely measure and evaluate the GPS-standardized time for the BTS transmitted signal at the communications frame boundary. The derived WCS BTS-to-GPS synchronization information of the present invention is used to support the enhanced performance of services that exploit the information obtained via precisely timed signal reception.
Examples of the background art for WCS protocols are available in the descriptions of the specified technologies for various wireless communications systems. E.g., a (North American, NA) Code Division Multiple Access (CDMA) WCS, as specified in EIA/TIA IS-95, operates with all of its BTSs simultaneously and individually synchronized to the GPS time base standard. Alternatively, BTSs deployed and operating in compliance with the 3GPP WCS technical specifications (TSs) for the Global System for Mobile communications (GSM) or the Universal Mobile Telecommunications System (UMTS) do not require, and do not typically implement, any such equivalent inter-BTS co-synchronization. Rather, the 3GPP specifications merely require the independent time base oscillator or clock for each cell sector to operate at a nominal specified standard frequency, to within the required tolerance for the standard BTSs.
Examples of the background art for the complimentary augmentation of WCS facilities are available in the descriptions of systems that determine the accurate location of the MS/UEs operating in standard configuration under normal WCS protocols and procedures. TruePosition, Inc., the assignee of the present invention, and its affiliates have been developing and deploying such wireless location system (WLS) facilities for many years, and a list below describes a multitude of its related patented technologies. As examples of related background art, the station-based WLS processing described in U.S. Pat. No. 6,285,321 and the matched-replica WLS signal processing described in U.S. Pat. No. 6,047,192 both employ correlative signal TOA determinations through the signal processing and analysis of uplink signals transmitted by MS/UEs. Additionally U.S. Pat. No. 6,388,618 describes the use of GPS-synchronized WLS location measurement units (LMUs), called therein signal collection systems (SCSs), for correlated analysis of uplink MS/UE signals. Further U.S. Pat. No. 6,351,235 thoroughly describes methods, procedures, and mechanisms for establishing a geographically distributed network of (WLS) local measurement units (LMUs or SCSs) that are all precisely and accurately synchronized to the GPS time base standard with an enhanced degree of stability. Finally U.S. Pat. No. 6,782,264 presents facilities for monitoring WCS A-bis interface messaging to cue and support WLS location determination operations. The descriptions for this A-bis Monitoring System (AMS) also suggest a cooperative mechanism for approximate BTS synchronization, through exploitation of the signal processing facilities that are inherent for the infrastructure-based WLS's processing of the uplink signals transmitted by MS/UEs.
The technology of the present invention provides precise and accurate synchronization information for any BTS of sufficiently detectable downlink signal strength, to within expected (e.g., Cramer-Rao and/or Ziv-Zakai) signal-processing accuracy bounds/limits. This technology applies the GPS-based synchronization implemented among a (WLS) deployed network of local signal measurement units to provide the common precise time base for the determination of the GPS-time base registration or relationship to BTS downlink transmissions detectable among the WCS network stations. The resultant precisely determined BTS time base relationship to the GPS time standard enables synchronized AGPS support to GPS-equipped MS/UEs, as well as synchronized MS/UE (uplink) signal data collection, with enhanced accuracy and efficiency for optimal WLS performance. Through the cost-effective application of the technology of the present invention, an adapted form of WLS synchronized signal data collection facilities provides the inventive derivation of the relations between the BTS downlink communications signal timebases and the GPS time standard. No uplink correlative signal analyses for timing determinations need be applied.
None of the background art descriptions of WCS or WLS technologies teach, provide, or describe the technology for downlink signal reception, evaluation, and analysis for the derivation of WCS BTS-to-GPS synchronization information. In contrast with the background art, including that referenced above, the technologies disclosed below integrate and exploit the GPS-synchronized collection and evaluation of BTS downlink signals to optimally determine, provide, and exploit the measured time relationship between the communications signal timebases, as observed for BTS frame boundaries, and the worldwide GPS time standard. Through the adaptations described below, the inventive methods and procedures presented herein can be integrated into, and applied in augmentation of and conjunction with, WLS facilities that are intended to be involved in the processing and analysis of uplink MS/UE transmitted signals. In such a multi-use configuration, the implementation and deployment of the present invention is particularly cost effective in its sharing of the applicable signal and data processing resources.